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"Results closest to experiment   are obtained with B3PW91, reproducing the experimental planar structure of pyrrole (Py) with the smallest error of 1.0%. The B3PW91 approach is then adopted, which additionally allows for comparison with our previous studies   . The Py molecule is a C 2v planar structure with a strong dipole moment of 1.94D in the direction of the NAH bond and À23.5, À26.8, À34.4D Å for the diagonal elements of the quadrupole matrix. "
[Show abstract][Hide abstract] ABSTRACT: A series of oligopyrrole dications, n-Py2+n-Py2+, n = 1–12, 15, 18, 21, 24, 27 are studied with hybrid density functional theory at the B3PW91/6-31+G(d,p) level. Geometry optimization and electronic structure of oligopyrrole dications are revisited using spin-restricted and spin-unrestricted methods. It is found that the ground states of short oligopyrrole dications n-Py2+n-Py2+ (n < 7) are spin-restricted singlet ground states. Thus, the charge carrier in these molecular dications has characteristics of a bipolaron. In contrast, medium size oligopyrrole dications (7⩽n⩽15)(7⩽n⩽15) have a spin-unrestricted singlet ground state and display charge carriers that have a polaron pair character. With increasing oligomer length, the spin-unrestricted singlet and the triplet become degenerate such that for even longer oligopyrrole dications (n-Py2+,n⩾18)(n-Py2+,n⩾18) the ground state is a triplet and the charge carrier is predominantly a polaron pair.
[Show abstract][Hide abstract] ABSTRACT: A classical model potential to simulate pyrrole oligomers in condensed phases is developed in this work. The new potential contains ten parameters that are optimized on a database of energy points calculated within the density functional theory approach. Based on this potential the condensed phase of systems composed of pyrrole oligomers with 4 and 12 monomers is studied as a function of system density. The binding energy, end-to-end distance, radius of gyration, vector and orientational order parameters, and pair correlation functions are reported at T=300 K. The mechanical equilibrium density is determined for both systems. The bulk modulus is reported at these densities, showing that systems composed of short oligomers are softer than systems containing longer oligomers. Analysis of pair correlation functions and order parameters indicates that at equilibrium the system of short oligomers has characteristics of a liquid while the system of longer oligomers shows a chain stacking trend.
The Journal of Chemical Physics 07/2010; 133(3):034905. DOI:10.1063/1.3457675 · 2.95 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Six isomers of dipicolinic acid (DPA) in the gas phase are predicted based on a detailed density functional theory study. Energetics include the binding energy, ionization potential, and electron affinity. Dipole moments, rotational constants, atomic charges, and Fukui functions are also calculated. Several isomerization reactions are shown as possible, with calculated reaction path and transition barriers in the range 6–14kcal/mol. Prediction of five DPA dimers and three crystalline DPA polytypes are also reported. Vibrational analysis for monomers and dimers is calculated and compared to existing experimental results. The electronic band structure of three crystalline polytypes shows that DPA crystals are insulators. The three stable polytypes display distinct hydrogen-bonding formations that originate sheets of DPA molecules that are further stacked to form layered crystals that are energetically nearly degenerate.